An integrated circuit (IC) may contain thousands of transistors, gates, latches, memory units etc., and a very large scale integrated (VLSI) circuit may contain a million or more active on a single silicon chip. The elements are organized into complex circuits of different kinds to serve a wide variety of applications. Some of these circuits, for proper operation when power to them is turned on, require that elements of the circuit be set to predetermined conditions or patterns of memory settings, by a process termed "initializing". Such a process is well known in the art and, for example, is accomplished by initially setting a number of "latches" (memory cells) to predetermined settings in a desired initial pattern. The initial setting of such latches must be carried out each time power to the circuits is again turned on after being off for even a brief instant.
To determine when power supply voltage to an IC comes on and has reached stable, sufficient operating level, circuits to detect when power is on have previously been developed. For example, when power is first turned on and as the supply voltage is beginning to rise from zero, a power-on detection circuit senses this condition and immediately generates a short pulse even before the supply voltage has reached full level. This short pulse in turn is used to generate a longer, somewhat delayed power-on signal (PWRON) which causes latches coupled thereto to be properly "initialized" to their predetermined settings. Thereafter, with a supply voltage at stable, full operating level the latches continuously send the information now stored in them to other circuits (i.e., the main circuits of the IC) as part of their operating program. When power is turned off, even for an instant, the latches can lose their settings, and must be reset (initialized) when power is restored. Failure to properly set (or reset) the latches results in erroneous or incorrect operation of the main circuits, as is well known.
When, for example, equipment (e.g., a large computer) is first turned on there may occur sudden negative voltage transients as various circuits within the equipment are powered up at slightly different times. Such negative voltage transients are random and can occur anytime after power is turned on. When such a negative voltage transient does occur, it can cause the latches to lose their proper settings. Unless the latches are immediately reset, the main circuits will not operate properly in the event power is immediately restored. Previous power-on detection circuits have been unable to respond quickly enough to such sudden negative voltage transients and so latches remained improperly set when power came on again.
It is desirable to have a power-on detection circuit which responds almost instantly to a power-off condition and is well adapted for fabrication on a VLSI circuit chip along with the main circuits on the chip.